Re: We really ought to do something about O_DIRECT and data=journalled on ext4

> Mac OS X: Like Solaris, there's a similar mechanism but it's not
> O_DIRECT; see
> http://stackoverflow.com/questions/2299402/how-does-one-do-raw-io-on-mac-os-x-ie-equivalent-to-linuxs-o-direct-flag
> for notes about the F_NOCACHE feature used. Same basic situation as
> Solaris; there's an API, but PostgreSQL doesn't use it yet.

Actually, on OSX 10.5.8, o_dsync and fdatasync aren't even available.
>From my run, it looks like even so regular fsync might be better than
open_sync. Results from a MacBook:

Sidney-Stratton:fsync josh$ ./test_fsync
Loops = 10000

Simple write:
8k write 2121.004/second

Compare file sync methods using one write:
(open_datasync unavailable)
open_sync 8k write 1993.833/second
(fdatasync unavailable)
8k write, fsync 1878.154/second

Compare file sync methods using two writes:
(open_datasync unavailable)
2 open_sync 8k writes 1005.009/second
(fdatasync unavailable)
8k write, 8k write, fsync 1709.862/second

Compare open_sync with different sizes:
open_sync 16k write 1728.803/second
2 open_sync 8k writes 969.416/second

Test if fsync on non-write file descriptor is honored:
(If the times are similar, fsync() can sync data written
on a different descriptor.)
8k write, fsync, close 1772.572/second
8k write, close, fsync 1939.897/second

--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com